CN1190322A - organic electroluminescent element - Google Patents

Abstract

An organic electroluminescence element, which can reduce the energy barrier encountered when injecting electrons from the cathode to the organic compound layer, and can realize a low driving voltage independent of the work function of the cathode material. Its composition is that the organic compound layer connected to the cathode 6 is made into a metal-doped layer 5 doped with a metal having the function of a donor (electron-donating) dopant, thereby reducing electron injection from the cathode to the organic compound layer. When encountering potential barriers and can reduce the driving voltage.

Description

organic electroluminescent element

The present invention relates to a class of organic electroluminescence elements (hereinafter referred to as organic EL elements) that can be used as planar light sources or display elements.

An organic electroluminescent device in which a light-emitting layer is composed of an organic compound is attracting attention as a large-area display device capable of driving at a low voltage. In order to increase the efficiency of the device, Tang et al. layered some organic compounds with different carrier transmission properties to form a structure that can inject holes and electrons through the anode and cathode in a balanced manner, and the film thickness of the organic layer Made below 2000 Å, as a result, a very practical high brightness and high efficiency with a performance of 1000cd/ ^m2 and an external quantum efficiency of 1% under the condition of an applied voltage below 10V has been successfully obtained [Applied Physics Communications (Appl. Phys. Lett., 51, 913 (1987)].

In such high-efficiency devices, energy barriers are generally encountered when injecting electrons through metal electrodes into organic compounds that are basically considered as insulators. In order to reduce this energy barrier, Tang et al. Function small Mg (Magnesium). Since Mg is easily oxidized and unstable, and has poor adhesion to the surface of organic matter, in this case, Mg is vapor-deposited together with Ag (silver), which is relatively stable and has good adhesion to the surface of organic matter. Use after melting.

Toppan Printing Co., Ltd. Group (51st Society of Applied Physics Lectures, Lecture Collection 28a-PB-4, p.1040) and Pioneer Co., Ltd. Group (54th Society of Applied Physics Academic Lectures, Lecture Collection 29p- ZC-15, p.1127) uses a substance stabilized by alloying Ti (lithium) with Al (aluminum) which has a smaller work function than Mg as the cathode, thereby achieving a higher level than the use of Mg alloys. The components have lower driving voltage and higher luminance. In addition, the present inventors have reported that a very thin Li layer of only about 10 Å is evaporated on the organic compound layer, and then a layer of silver is laminated on it, so that the two-layer cathode can effectively low driving voltage [IEEE Trans. Electron Devices., 40, 1342 (1993)].

Also recently, Pei et al. of UNIAX successfully lowered the driving voltage by doping Li salt in the polymer light-emitting layer [Science (Science), 269, 1086 (1995)]. In this way, the Li salt dispersed in the polymer light-emitting layer is dissociated by the action of the applied voltage, so that the Li ions and their counter ions are respectively distributed near the cathode and the anode, so that the polymer molecules near the electrode can be in situ become adulterated substances. In this case, since the polymer near the cathode is reduced by Li as a donor (electron-donating) dopant, it exists in the state of free anions, so the potential barrier when injecting electrons from the cathode is lower than that of a polymer without Li doping. The case is much lower [Science (Science), 269, 1086 (1995)].

However, even in the alloy electrode of Mg and Li, the element will be deteriorated due to the action of electrode oxidation, etc., so the function as a wiring material must be considered, and the selection of electrode material is limited when it is used as an alloy electrode. In the two-layer cathode of the present inventors, when the thickness of the Li layer is more than 20 Å, it does not function as a cathode [IEEE Trans. Electron Devices.), 40, 1342 (1993)]. In addition, for the method of adding salt to the light-emitting layer by Pei et al. for in-situ doping by the dissociation effect caused by the electric field, the movement time of the dissociated ions reaching the vicinity of the electrode becomes the dominant speed, so that the response speed of the element is very slow , which is its shortcoming.

In view of the above circumstances, the present invention aims to realize a low driving voltage independent of the work function of the cathode material by reducing the energy barrier encountered when injecting electrons from the cathode into the organic compound layer.

Another object of the present invention is to provide such an element, even when such an element is used alone as a cathode material, such as Al, which has been generally used as a wiring material in the past and is cheap and stable, can exhibit the advantages of using the above-mentioned alloy as an electrode. same or better performance.

The present inventors have found that if a metal having a donor (electron-donating) dopant function is used to dope the organic compound layer connected to the cathode, the potential barrier for injecting electrons from the cathode to the organic compound layer can be reduced, The driving voltage can thereby be lowered, and the present invention has been accomplished due to this finding.

That is to say, the organic EL element of the present invention is an organic EL element having at least one light-emitting layer made of an organic compound between the anode and the cathode facing each other, and is characterized in that, in the organic EL element, At the interface with the cathode, there is a layer of an organic compound doped with a metal having the function of a donor dopant as a metal-doped layer.

The donor dopant, more specifically, may be composed of any one or more metals among transition metals including alkali metals, alkaline earth metals, and rare earth metals having a work function of 4.2 eV or less. In addition, the concentration of the dopant in the metal doped layer is preferably 0.1 to 99% by weight, and the thickness of the metal dopant layer is preferably 10 Å to 3000 Å.

Fig. 1 is a schematic view showing an embodiment of the organic EL element of the present invention. Its structure is that on a glass substrate (transparent substrate) 1, a transparent electrode 2 constituting an anode, a hole transporting layer 3 having hole transporting properties, a light-emitting layer 4, a metal doped layer 5 and a cathode constituting a cathode are laminated in sequence. The back electrode 6. Among these elements (layers), glass substrate (transparent substrate) 1, transparent electrode 2, hole transport layer 3, light-emitting layer 4 and cathode 6 are all well-known elements, while metal-doped layer 5 is proposed by the present invention. elements (layers). As a specific stacked structure of the organic EL element, in addition to the above schemes, it is also possible to include: anode/light emitting layer/metal doped layer/cathode, anode/hole transport layer/light emitting layer/metal doped layer/cathode/, Anode / hole transport layer / light emitting layer / electron transport layer / metal doped layer / cathode, anode / hole injection layer / light emitting layer / metal doped layer / cathode, anode / hole injection layer / hole transport layer / Light emitting layer/metal doped layer/cathode, anode/hole injection layer/hole transport layer/light emitting layer/electron transport layer/metal doped layer/cathode and other schemes, but the EL element of the present invention, as long as it is in the same There is a metal doped layer 5 at the interface of the cathode 6, and it can be composed of any kind of element.

In the organic EL element, the process of injecting electrons from the cathode to the organic compound layer which is basically an insulator is a reduction process of the organic compound on the surface of the cathode, that is, the process of forming a free anion state [Phys.Rev.Lett ., 14, 229 (1965)]. In the element of the present invention, by doping in advance the metal as the donor (electron-donating) dopant substance that can become the reducing agent of the organic compound into the organic compound layer in contact with the cathode, it is possible to reduce the injection rate from the cathode. Energy barriers in electrons. The metal-doped layer 5 is such an organic compound layer so doped with a metal functioning as a donor dopant. In the metal-doped layer, since there are molecules that have been reduced by the dopant and are in a reduced state (that is, a state in which electrons have been accepted and electrons have been injected), the energy barrier for electron injection is small, which is different from the previous organic EL. The drive voltage can be lowered compared to components. Furthermore, a stable metal such as Al, which is generally used as a wiring material, can be used for the cathode.

In this case, the donor dopant is not particularly limited as long as it contains an alkali metal such as Li, an alkaline earth metal such as Mg, or a transition metal of a rare earth metal capable of reducing an organic compound. . Among them, metals with a work function of 4.2 eV or less are particularly suitable, and specific examples include Li, Na, K, Be, Mg, Ca, Sr, Ba, Y, La, Mg, Sm, Gd, and Yb.

The dopant concentration in the metal-doped layer is not particularly limited, but is preferably 0.1 to 99% by weight. If it is less than 0.1% by weight, the concentration of molecules reduced by the dopant (hereinafter referred to as reduced molecules) is too low, and its doping effect is small. If it exceeds 99% by weight, the metal concentration in the film far exceeds that of organic molecules. Concentration, so that the concentration of reducing molecules is very low, so the doping effect is not good. In addition, the thickness of the metal-doped layer is not particularly limited, but is preferably 10 Å to 3000 Å. If it is less than 10 Å, the number of reducing molecules present near the electrode interface is very small, and the doping effect is poor. If it exceeds 3000 Å, the film thickness of the entire organic layer is too thick, resulting in an increase in the driving voltage, so it is not possible to good.

The film-forming method of the above-mentioned metal-doped layer 5 may be any thin film forming method, for example, an evaporation method or a sputtering method may be used. In addition, when a thin film can be formed by coating a solution, a solution coating method such as a spin coating method or a dip coating method can be used. In this case, it can be used by dispersing the organic compound to be doped together with a dopant in an inert polymer.

The organic compound that can be used in the light-emitting layer, the electron transport layer, and the metal-doped layer is not particularly limited, but examples include: polycyclic compounds such as p-terphenyl and p-quaterphenyl, and derivatives thereof; naphthalene, tetrahydroanthracene , pyrene, coronet, , anthracene, diphenylanthracene, naph-thacene, phenanthrene and other condensed polycyclic hydrocarbons and their derivatives; phenanthroline, red phenanthroline, phenanthridine, acridine , quinoline, quinoxaline, phenazine and other condensed heterocyclic compounds and derivatives thereof; fulorosein, perylene, phthaloperylene, naphthoperylene, perikinone, phthaloperylene, naphthoperylene, Diphenylbutadiene, tetraphenylbutadiene, oxadiazole, aldehyde azine, dibenzoxazine, distyryne, pyrazine, cyclopentadiene, 8-hydroxyquinoline, aminoquinoline , imine, stilbene, vinylanthracene, diaminocarbazole, pyran, thiopyran, polymethine, phenolcyanine, quinacridone, rubrene, etc. and their derivatives.

In addition, the metal chelate complexes disclosed in JP-A-63-295695, JP-8-22557, JP-8-81472, JP-5-9470, JP-5-17764 Among compounds, especially in metal chelated oxanoid compounds, tris(8-hydroxyquinolyl)aluminum, bis(8-hydroxyquinolyl)magnesium, bis[benzo(f)-8- Hydroxyquinolyl]zinc, bis(2-methyl-8-hydroxyquinolyl)aluminum, tris(8-hydroxyquinolyl)indium, tris(5-methyl-8-hydroxyquinolyl)aluminum, 8-hydroxyquinolyl lithium, tris(5-chloro-8-hydroxyquinolyl) potassium, bis(5-chloro-8-hydroxyquinolyl) calcium, etc. have at least one 8-hydroxyquinolyl group or its derivatives compounds as metal complexes with ligands.

Oxadiazoles disclosed in JP-A-5-202011, JP-A-7-179394, JP-A-7-278124, JP-A-7-228579, and JP-A-7-157473 Triazines, stilbene derivatives disclosed in JP-A-6-203963 and distyrylpropyne derivatives, styrenes disclosed in JP-A-6-132080 and JP-A-6-88072 As derivatives, the diolein derivatives disclosed in JP-A-6-100857 and JP-A-6-207170 are also suitable for use as light-emitting layers, electron transport layers, and metal-doped layers.

In addition, fluorescent whitening agents such as benzoxazoles, benzothiazoles, and benzimidazoles can also be used, and examples thereof include compounds disclosed in JP-A-59-194393. Representative examples thereof include 2,5-bis(5,7-di-tert-amyl-2-benzoxazolyl)-1,3,4-thiazole, 4,4'-bis(4, 7-di-tert-amyl-2-benzoxazolyl)stilbene, 4,4'-bis[5,7-bis(2-methyl-2-butyl)-2-benzoxazolyl]stilbene , 2,5-bis(5,7-di-tert-amyl-2-benzoxazolyl)thiophene, 2,5-bis[5-(α,α-dimethylbenzyl)-2-benzo Oxazolyl]thiophene, 2,5-bis[5,7-bis(2-methyl-2-butyl)-2-benzoxazolyl]-3,4-diphenylthiophene, 2,5 -bis(5-methyl-2-benzoxazolyl)thiophene, 4,4'-bis(2-benzoxazolyl)biphenyl, 5-methyl-2-{2[4-(5 -Methyl-2-benzoxazolyl)phenyl]vinyl}benzoxazole, 2-[2-(4-chlorophenyl)vinyl]naphtho(1,2-d)oxazole, etc. Benzoxazoles, 2,2′-(p-phenylene divinylene)-bisbenzothiazoles, etc., 2-{2-[4-(2-Benzimidazolyl) Fluorescent whitening agents such as benzimidazoles such as phenyl]vinyl}benzimidazole and 2-[2-(4-carboxyphenyl)vinyl]benzimidazole.

As the distyrylbenzene compound, for example, a compound disclosed in European Patent No. 0373582 can be used. Representative examples thereof include: 1,4-bis(2-methylstyryl)benzene, 1,4-bis(3-methylstyryl)benzene, 1,4-bis(4-methylstyryl) styryl)benzene, distyrylbenzene, 1,4-bis(2-ethylstyryl)benzene, 1,4-bis(3-ethylstyryl)benzene, 1,4-bis (2-methylstyryl-2-methylbenzene, 1,4-bis(2-methylstyryl)-2-ethylbenzene, etc.

In addition, the distyrylpyrazine derivatives disclosed in JP-A-2-252793 can also be used as light-emitting layers, electron-transporting layers, and metal-doped layers. Representative examples thereof include 2,5-bis(4-methylstyryl)pyrazine, 2,5-bis(4-ethylstyryl)pyrazine, 2,5-bis[2 -(1-naphthyl)vinyl]pyrazine, 2,5-bis(4-methoxystyryl)pyrazine, 2,5-bis[2-(4-biphenyl)vinyl]pyrazine oxazine, 2,5-bis[2-(1-pyrenyl)vinyl]pyrazine, etc.

In addition, dimethylene derivatives disclosed in European Patent No. 388768 and JP-A-3-231970 can also be used as materials for light-emitting layers, electron transport layers, and metal-doped layers. Representative examples thereof include: 1,4-phenylene dimethylene, 4,4'-phenylene dimethylene, 2,5-xylylene dimethylene, 2,6-naphthylene dimethylene , 1,4-diphenylene dimethylene, 1,4-p-phenylene dimethylene, 9,10-anthracenediyl dimethylene, 4,4'-(2,2-di-tert-butylphenylethylene base) biphenyl, 4,4'-(2,2-diphenylvinyl)biphenyl, etc., and derivatives thereof, silylamines disclosed in JP-A-6-49079 and JP-A-6-293778 Derivatives, polyfunctional styrene compounds disclosed in JP-A-6-279322 and JP-A-6-279323, oxadiazoles disclosed in JP-A-6-107648 and JP-A-6-92947 Derivatives, anthracene compounds disclosed in JP-A-6-206865, Oxynate derivatives disclosed in JP-A-6-145146, tetraphenylbutadiene compounds disclosed in JP-A-4-96990 , the organic trifunctional compound disclosed in JP-A-3-296595, and the coumarin derivative in JP-A-2-191694, the perylene derivative disclosed in JP-A-2-196885, in The naphthalene derivatives disclosed in JP-A-2-255789, the phthaloperioinone derivatives disclosed in JP-A-2-289676 and JP-A-2-88689, and the phthaloinone derivatives disclosed in JP-A-2-2509292 styrylamine derivatives, etc.

In addition, known compounds conventionally used in the production of organic EL elements are also suitably used.

The arylamine compounds that can be used in the hole injection layer, the hole transport layer, and the hole transport light-emitting layer are not particularly limited, but are preferably listed in JP-A-6-25659 and JP-A-6-203963. Gazettes, JP-A-6-215874, JP-7-145116, JP-7-224012, JP-7-157473, JP-8-48656, JP-7-126226, JP-A-7-188130, JP-8-40995, JP-8-40996, JP-8-40997, JP-7-126225, JP-7-101911, JP-A Specific examples of the arylamine compounds disclosed in Publication No. 7-97355 are: N,N,N'N'-tetraphenyl-4,4'-diaminobiphenyl, N,N'-diphenyl -N,N'-bis-(3-methylphenyl)-4,4'-diaminobiphenyl, 2,2-bis(4-di-p-tolylaminophenyl)propane, N,N,N 'N'-tetra-p-tolyl-4,4'-diaminobiphenyl, bis(4-di-p-tolylaminophenyl)phenylmethane, N,N'-diphenyl-N,N'- Bis(4-methoxyphenyl)-4,4'-diaminobiphenyl, N,N,N'N'-tetraphenyl-4,4'-diaminodiphenyl ether, 4,4'- Bis(diphenylamino)tetraphenyl, 4-N,N-diphenylamino-(2-diphenylvinyl)benzene, 3-methoxy-4'-N,N-diphenylaminobenzene Vinylbenzene, N-phenylcarbazole, 1,1-bis(4-bis-p-triaminophenyl)-cyclohexane, 1,1-bis(4-bis-p-triaminophenyl)-4 -Phenylcyclohexane, bis(4-dimethylamino-2-methylphenyl)-phenylmethane, N,N,N-tris(p-tolyl)amine, 4-(two-p-tolylamino )-4'-[4-(two-p-tolylamino)styryl]stilbene, N,N,N'N'-tetra-p-tolyl-4,4'-diamino-biphenyl, N, N,N'N'-tetraphenyl-4,4'-diamino-biphenyl-N-phenylcarbazole, 4,4'-bis[N-(1-naphthyl)-N-phenyl -amino]biphenyl, 4,4"-bis[N(-1-naphthyl)-N-phenyl-amino]-terphenyl, 4,4'-bis[N-(2-naphthyl)-N -Phenyl-amino]biphenyl, 4,4'-bis[N-(3-acenaphthyl)-N-phenyl-amino]biphenyl, 1,5-bis[N-(1-naphthyl)- N-phenyl-amino]naphthalene, 4,4′-bis[N-(9-anthracenyl)-N-phenyl-amino]biphenyl, 4,4″-bis[N-(1-anthracenyl) -N-phenyl-amino] p-terphenyl, 4,4'-bis[N-(2-phenanthrenyl)-N-phenyl-amino]biphenyl, 4,4'-bis[N-(8- Fluoranthenyl)-N-phenyl-amino]biphenyl, 4,4'-bis[N-(2-pyrenyl)-N-phenyl-amino]biphenyl, 4,4'-bis[N- (2-perylenyl)-N-phenyl-amino]biphenyl, 4,4'-bis[N-(1-comonyl)-N-phenyl-amino]biphenyl, 2,6-bis(di p-tolylamino)naphthalene, 2,6-bis[bis(1-naphthyl)amino]naphthalene, 2,6-bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene , 4,4-bis[N,N-bis(2-naphthyl)amino]terphenyl, 4,4'-bis{N-phenyl-N-[4-(1-naphthyl)phenyl] Amino}biphenyl, 4,4'-bis[N-phenyl-N-(2-pyrenyl)-amino]biphenyl, 2,6-bis[N,N-bis(2-naphthyl)amino] Fluorene, 4,4"-bis(N,N-xylylamino) terphenyl, bis(N-1-naphthyl)(N-2-naphthyl)amine, etc. In addition, known compounds conventionally used in the production of organic EL elements are also suitably used.

In addition, as the hole injection layer, the hole transport layer, and the hole transport light-emitting layer, those obtained by dispersing the above-mentioned organic compound in a polymer or polymerized can also be used. In addition, δ-conjugate polymerization of so-called vene-conjugated polymers such as poly(p-styrene) or its derivatives, hole-transporting non-conjugated polymers represented by poly(N-vinylcarbazole), and polysilanes can also be used.

The hole injection layer formed on the ITO electrode is not particularly limited, but metal phthalocyanines such as copper phthalocyanine, metal-free phthalocyanines, carbon films, and conductive polymers such as polyaniline are preferably used. In addition, it can also be used as a hole injection layer by reacting a Lewis acid as an oxidizing agent with the above-mentioned arylamines to form free cations.

The cathode is not limited as long as it is a metal that can be used stably in air, but aluminum, which is generally widely used as a wiring electrode, is particularly preferable.

[Example]

The following examples are given to explain the present invention, but the present invention is not limited to these examples. In addition, for vapor deposition of organic compounds and metals, a VPC-400 vacuum vapor deposition machine manufactured by Vacuum Kiko was used, and for spin coating, an IH-D3 type spin coater manufactured by Mikasa Corporation was used. For the measurement of the film thickness, a DekTak 3ST type stylus height difference meter manufactured by Slo-In Co., Ltd. was used.

Jusui PBX40-2.5 DC power supply, Yantong VOAC-7510 multimeter and Topcon BM-8 luminance meter were used in the characteristic evaluation of components. With the ITO of the element as the anode and Al as the cathode, the DC voltage is gradually increased according to the ratio of IV/2 seconds, and the brightness and current value after the voltage rises for 1 second are measured at the same time. In addition, the determination of the EL spectrum was carried out using a Hamamatsu Hotnix PMA-10 optical multi-channel analyzer under constant current drive.

Example 1

The present invention is applied to an organic EL element having a laminated structure shown in FIG. 1 . An anode transparent electrode 2 is coated on a glass substrate 1, and the anode is ITO (indium-tin oxide, electron beam vapor deposition product manufactured by Asahi Glass Co., Ltd.) having a sheet resistance of 15Ω/□. On the anode, according to 10 ^-6 torr and 3 Å/sec vapor deposition rate, a layer of hole transport property is formed by the following formula I: The αNPD layer shown was formed into a film with a thickness of 400 Å, thereby forming the hole transport layer 3 .

Next, on the above-mentioned hole transport layer 3, according to the same conditions as the hole transport layer 3, a layer of green luminescence with a thickness of 300 Å is vacuum-evaporated by the following formula 2: The tris(8-hydroxyquinolyl)aluminum complex layer (hereinafter referred to as “Al _q ”) 4 is represented, thereby forming the light emitting layer 4 .

Next, a metal-doped layer 5 is formed on the above-mentioned light-emitting layer 4, which contains _Alq and Li, and the respective evaporation rates are adjusted during evaporation so that Li accounts for 1.5% by weight, thus forming a film of 400 Å. .

Finally, a layer of Al with a thickness of 1000 Å is evaporated on the above metal doped layer 5 at an evaporation rate of 15 Å/sec, so as to form the back electrode 6 as the cathode. The light-emitting area is a square with a length of 0.5 cm and a width of 0.5 cm.

In the above-mentioned organic EL element, a DC voltage is applied between ITO as an anode and Al6 as a cathode. The brightness of the green light emitted by the light-emitting layer Al _q 4 was measured. The dotted curves in Fig. 2 and Fig. 3 represent the luminance-voltage characteristics and luminance-current density characteristics respectively, and the highest luminance of 39000cd/m ² is produced at 12V. The current density at this time was 800 mA/em ² . In addition, a brightness of 1000 cd/m ² was obtained at 8V.

Comparative example 1

In the same manner as in Example 1, a layer of αNPD film with a thickness of 400 Å was first formed on the ITO as a hole transport layer, and then a layer with a thickness of αNPD was formed on this layer by vacuum evaporation under the same conditions as the hole transport layer An _Alq film of 600 Å was used as the light emitting layer. Then, an Al film with a thickness of 2000 Å is deposited on the Al _q layer as a cathode.

The triangular point curves in Fig. 2 and Fig. 3 represent the luminance-voltage characteristics and luminance-current density characteristics of the component respectively, and only the highest luminance of 6700cd/ ^m2 is produced at 15V. In addition, in order to obtain a brightness of 1000cd/m ² , a voltage of 13V must be applied. From this experimental result, it can be seen that the metal-doped layer 5 is effective for lowering the driving voltage.

Comparative example 2

According to the same conditions as in Example 1, a layer of αNPD film with a thickness of 400 Å was first formed on the ITO as a hole transport layer, and then a layer was formed by vacuum evaporation on this layer according to the proportion of Li accounting for 1.5% by weight. _Alq and Li films with a thickness of 300 Å, and then separately vapor-deposit a layer of Al _q film with a thickness of 300 Å. Then a layer of 1000 Å of Al is vapor-deposited on the Al _q as the cathode.

The component produces only a maximum luminance of 8cd/ ^m2 at 25V. This result indicates that the presence of a Li-doped Al _q layer near the cathode is essential for achieving high luminance. In addition, the spectrum emitted by the element is broader than the original spectrum of Al _q and reduces its fluorescence. This fact indicates that Al _q is reduced by doped Li, thereby changing the energy level of Al _q . This can also be confirmed from the UV and visible spectra of Li-doped _Alq films.

Comparative example 3

In the same manner as in Example 1, a layer of αNPD film with a thickness of 400 Å was first formed on the ITO as a hole transport layer, and then a layer with a thickness of αNPD was formed on this layer by vacuum evaporation under the same conditions as the hole transport layer An _Alq film of 600 Å was used as the light emitting layer. Then, a layer of Mg and Ag with a thickness of 1500 Å is evaporated on the Al _q layer according to the weight ratio of Mg and Ag being 10:1.

The element produces a maximum brightness of 17000cd/ ^m2 at 13V. In addition, in order to obtain a brightness of 1000cd/m ² , a voltage of 9.5V must be applied. For the element with the metal-doped layer of Example 1, the voltage is only 8V, so it can be considered that, compared with the alloy cathode, the element with the metal-doped layer can reduce the driving voltage, and its maximum brightness is also higher .

Example 2

表示的红菲咯啉和Li，使其中的Li浓度为2重量％，将其作为金属掺杂层5。然后再在该层上蒸镀一层1000厚的Al作为阴极6制成元件，该元件在施加电压12V时产生最高辉度28000cd/m²和电流密度8200mA/cm²仅仅用，与实施例1同样地以低的驱动电压获得高的亮度。On ITO, one deck of 400 Å thick αNPD is vacuum evaporated as the hole transport layer 3, and one deck of 300 Å thick _Alq is vacuum evaporated as the light-emitting layer 4, and then a total of 300 Å thick αNPD is evaporated by the following formula 3:

The bathophenanthroline and Li shown in FIG. Then vapor-deposit a layer of 1000 Å thick Al on this layer as the cathode 6 to make the element, the element produces the highest luminance of 28000cd/ ^m2 and current density of 8200mA/ ^cm2 when the applied voltage of 12V is only used, and the embodiment 1 Similarly, high brightness can be obtained with low driving voltage.

Comparative Example 5

A layer of 400 Å thick αNPD was vacuum evaporated on ITO as a hole transport layer, a layer of 300 Å thick Al _q was vacuum evaporated as a light emitting layer, and then only a layer of 300 Å thick bathophenanthroline was evaporated. A layer of 1000 Å of Al was vapor-deposited on it as a cathode to form an element. Applying a voltage of 15V to this element can only reach a current density of 270mA/cm ² and a maximum brightness of 9500cd/m ² , so it can be considered that in Example 4, due to the doping of Li to the bathophenanthroline layer, it can effectively reduce the drive voltage.

Example 3

A layer of 400 Å thick αNPD was vacuum evaporated on ITO as the hole transport layer 3, a layer of 600 Å thick Al _q was vacuum evaporated as the light emitting layer 4, and then a 20 Å thick layer of Al _q and Mg was co-evaporated. , making the Mg therein account for 93% by weight, as the metal-doped layer 5 . A 1000 Å thick layer of Al was vapor-deposited on this layer as the cathode 6, thus completing the element. This device can produce a maximum luminance of 28000 cd/m ² and a current density of 920 mA/cm ² , and can produce high luminance similarly to Example 1.

Example 4

According to the method of Burroughes et al. [Nature, 347, 539 (1990)], a 1000 Å thick polyparastyrene (PPV) film was formed on ITO as the light emitting layer 4 . A polystyrene with a molecular weight of 200,000 and diphenylanthracene is dissolved in tetrahydrofuran at a ratio of 2:1 by weight, and then 2% by weight of Li equivalent to diphenylanthracene is dispersed therein, and its Stir to react Li with diphenylanthracene. Using this tetrahydrofuran solution, spin-spray a layer of polystyrene film containing anthracene/Li on the above-mentioned PPV film in a nitrogen atmosphere, thereby forming a metal-doped layer 5 of 50 Å. A layer of Al with a thickness of 1000 Å is vapor-deposited on it as the cathode 6 . The element can observe yellow-green light emitted from its PPV layer, and can display a maximum brightness of 4200cd/ ^m2 .

Comparative example 6

A 1000 Å thick PPV film was formed on the ITO in the same manner as in Example 4, and then a 1000 Å thick Al film was vapor-deposited thereon to form an element. The element was also observable as yellow-green light emanating from its PPV layer and exhibited a peak luminance of 400cd/ ^m2 . Therefore, it can be considered that the metal-doped layer in Embodiment 4 can effectively reduce the driving voltage.

As described above, in the organic EL element of the present invention, a donor (electron-donating) dopant metal is doped in the organic compound layer, and the metal-doped doped layer is provided at the interface with the cathode. Therefore, a light-emitting element with low driving voltage, high efficiency and high brightness can be made. Therefore, the organic EL element of the present invention has high utility, and can be expected to be effectively used as a display element and a light source.

FIG. 1 is a schematic cross-sectional view showing an example of a laminated structure of an organic EL device of the present invention.

Fig. 2 is a graph showing the luminance-voltage characteristics of the organic EL element of the present invention and a comparative example.

Fig. 3 is a graph showing the luminance-current density characteristics of the organic EL device of the present invention and a comparative example.

Explanation of symbols

1. Transparent substrate

2. Transparent positive plate

3. Hole transport layer

4. Luminescent layer

5. Metal doped layer

6. Cathode

Claims (5)

1. An organic electroluminescent element, which is an organic electroluminescent element having at least one light-emitting layer made of an organic compound between the anode and the cathode facing each other, characterized in that, in the element , has a layer of an organic compound doped with a metal having a function of a donor (electron-donating) dopant at the interface with the above-mentioned cathode. 2. The organic electroluminescent element according to claim 1, wherein the above-mentioned donor dopant in the element is made of alkali metal, alkaline earth metal and rare earth metal with a work function below 4.2eV. Consisting of one or more metals selected from the transition metals included. 3. The organic electroluminescence element according to claim 1 or 2, characterized in that, in the element, the concentration of the donor dopant in the metal-doped layer is 0.1-99% by weight. 4. The organic electroluminescence element according to any one of claims 1 to 3, characterized in that, in the element, the thickness of the metal-doped layer is 10 Å to 3000 Å. 5. The organic electroluminescence element according to any one of claims 1 to 4, wherein at least one of the materials constituting the cathode is aluminum.

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